President Barack Obama and Afghan President Hamid Karzai (AP Photo/Charles Dharapak)

(CNSNews.com) – The White House stressed Friday that it respected the religious beliefs and practices of the Afghan people, in explaining why President Barack Obama apologized to Afghan President Hamid Karzai for the burning of Qurans that prompted violence against American soldiers.

The White House announced Thursday that Obama wrote a letter of apology to Karzai. A few hours later, news organizations reported that an Afghan soldier had killed two U.S. troops and wounded others in retaliation for the Quran burning.

The two U.S. troops were killed during a protest Thursday outside the American base in the Khogyani district of Nangarhar province.

Thousand of Afghans took to the streets after midday prayers on Friday. At least 20 people, including the two U.S. soldiers, have been killed in four days of violence, the Associated Press reported.

A reporter asked White House spokesman Josh Earnest what the process was in deciding whether to make an apology, and whether such an action was appropriate. Earnest said the safety of U.S. soldiers in Afghanistan was one factor, but also stressed it was out of sincere regret.

Earnest responded that Obama; Gen. John Allen, the commander of U.S. forces in Afghanistan; and Defense Secretary Leon Panetta “have all in different forms expressed their apology on behalf of the American people and the American military to the Afghan people to articulate that the United States military and indeed the American people have enduring respect for the religious views and religious practices of the Afghan people.”

Further, Earnest said, it is important that Karzai has called for restraint from the Afghans.

“We were pleased today to also see that President Karzai himself has also called or appealed for calm in Afghanistan and while this is a difficult circumstance we’re working through in Afghanistan, we’re confident our goals in Afghanistan – which I’ll remind you – is to defeat, disrupt and dismantle al Qaeda and to ensure that al Qaeda can’t be used as a safe haven for al Qaeda and other religious extremists, violent extremists,” Earnest said. “So, it is our view that we will work through the difficult circumstances, and remain on track to making progress in our goals there.”

But protesters have ignored appeals by Karzai, parliamentarians and some clerics for an end to the violence until an investigation into the incident at Bagram Air Field is concluded in coming days. Afghan officials said seven people were killed around the country Friday by Afghan security forces trying to disperse crowds or responding to gunfire from protesters.

The violent protests began Tuesday, when Afghanis at the Bagram air base noticed Qurans and other Islamic texts were in the trash that coalition troops dumped into a pit where garbage is burned. Afghan government officials said initial reports indicated four Qurans were burned, the AP reported.

The materials were taken from a library at Parwan Detention Facility, which adjoins the base, because they contained extremist messages or inscriptions. Writing inside a Quran is forbidden in the Islamic faith, although it was unclear whether the handwritten messages were found in the holy book or other reading materials.

Republican candidates for president have criticized the president for making the apology.

Former House Speaker Newt Gingrich, at a campaign event in Spokane, Wash., Thursday said that Karzai should be apologizing for the two murdered U.S. soldiers.

“There seems to be nothing that radical Islamists can do to get Barack Obama's attention in a negative way, and he is consistently apologizing to people who do not deserve the apology of the president of the United States, period,” Gingrich said.

“And, candidly, if Hamid Karzai, the president of Afghanistan, doesn't feel like apologizing, then we should say good bye and good luck. We don't need to be here risking our lives and wasting our money on somebody who doesn't care,” he added.